Hot stamped body

ABSTRACT

A hot stamped body comprising a steel base material and an Al—Zn—Mg-based plating layer formed on a surface of the steel base material, wherein the plating layer has a predetermined chemical composition, the plating layer comprises an interfacial layer positioned at an interface with the steel base material and containing Fe and Al and a main layer positioned on the interfacial layer, the main layer comprises, by area ratio, 10.0 to 85.0% of an Mg—Zn containing phase and 15.0 to 90.0% of an Fe—Al containing phase, the Mg—Zn containing phase comprises at least one selected from the group consisting of an MgZn phase, Mg 2  Zn 3  phase, and MgZn 2  phase, and the Fe—Al containing phase comprises at least one of an FeAl phase and Fe—Al—Zn phase and an area ratio of the Fe—Al—Zn phase in the main layer is 10.0% or less.

FIELD

The present invention relates to a hot stamped body.

BACKGROUND

As a technique for press-forming a material which is difficult to shape,such as high strength steel sheet, hot stamping (hot pressing) is known.Hot stamping is a hot shaping technique which shapes a material suppliedfor shaping after heating it. In this technique, the material is shapedafter heating, therefore at the time of shaping, the steel material issoft and has good shapeability. Therefore, even a high strength steelmaterial can be precisely formed into a complicated shape. Further, thepress die simultaneously performs the shaping and hardening, thereforeit is known that after shaping, the steel material has sufficientstrength.

PTL 1 describes a plated steel sheet for hot pressing characterized byhaving an Al—Zn-based alloy plating layer containing Al: 20 to 95 mass%, Ca+Mg: 0.01 to 10 mass %, and Si on the steel sheet surface. Further,PTL 1 describes that such a plated steel sheet can prevent the platingfrom adhering to the die at the time of hot pressing, since oxides of Caor Mg are formed on the surface of the Al—Zn-based alloy plating layer.

In relation to an Al—Zn-based alloy plating, PTL 2 describes an alloyplated steel material characterized by containing, by mass %, Al: 2 to75%, Fe: 2 to 75%, and a balance of 2% or more of Zn and unavoidableimpurities in the plating layer. Further, PTL 2 teaches that, from theviewpoint of improvement of the corrosion resistance, it is effective tofurther include Mg: 0.02 to 10%, Ca: 0.01 to 2%, Si: 0.02 to 3%, etc.,in the plating layer.

Further, in relation to an Al—Zn-based alloy plating, PTL 3 describes aZn-based plated steel material for hot pressing having at a surface-mostlayer an oxide layer mainly comprising Zn and containing Mn in 1% ormore in mass %, having underneath that a plating layer comprising aZn-based alloy, and containing in the Zn-based plated layer one or moreof Ni: 0.01 to 20%, Cr: 0.01 to 10%, Mn: 0.01 to 10%, Mo: 0.01 to 5%,Co: 0.01 to 5%, Al: 0.01 to 60%, Si: 0.01 to 5%, Mg: 0.01 to 10%, Ca:0.01 to 5%, and Sn: 0.01 to 10%.

Further, PTL 4 describes a plated steel material comprising a steelmaterial and a plating layer arranged on the surface of the steelmaterial and containing a Zn—Al—Mg alloy layer, wherein the Zn—Al—Mgalloy layer has a Zn phase, the Zn phase contains an Mg—Sn intermetalliccompound phase, and the plating layer contains, by mass %, Zn: more than65.0%, Al: more than 5.0% to less than 25%, Mg: more than 3.0% to lessthan 12.5%, Ca: 0% to 3.00%, Si: 0% to less than 2.5%, etc.

Similarly, PTL 5 describes a plated steel material comprising a steelmaterial and a plating layer arranged on a surface of the steel materialand containing a Zn—Al—Mg alloy layer, wherein, in a cross-section ofthe Zn—Al—Mg alloy layer, an area ratio of an MgZn₂ phase is 45 to 75%,an area ratio of a total of the MgZn₂ phase and Al phase is 70% or more,an area ratio of a Zn—Al—MgZn₂ ternary eutectic structure is 0 to 5%,and the plating layer contains, by mass %, Zn: more than 44.90% to lessthan 79.90%, Al: more than 15% to less than 35%, Mg: more than 5% toless than 20%, Ca: 0.1% to less than 3.0%, Si: 0% to 1.0%, etc.

CITATIONS LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Publication No. 2012-112010-   [PTL 2] Japanese Unexamined Patent Publication No. 2009-120948-   [PTL 3] Japanese Unexamined Patent Publication No. 2005-113233-   [PTL 4] WO 2018/139619-   [PTL 5] WO 2018/139620

SUMMARY Technical Problem

If, for example, using a Zn-based plated steel material in hot stamping,the material is worked in a state where the Zn is molten, therefore themolten Zn will sometimes penetrate into the steel and cause crackinginside the steel material. Such a phenomenon is called “liquid metalembrittlement (LME)”. It is known that the fatigue characteristics of asteel material fall due to the LME.

On the other hand, if using a plated steel material containing Al as aconstituent of the plating layer in hot stamping, it is known that, forexample, the hydrogen generated at the time of heating in the hotstamping will sometimes penetrate the steel material and cause hydrogenembrittlement cracking.

However, in conventional Al—Zn-based plated steel materials used in hotstamping, there has not necessarily been sufficient study from theviewpoint of suppressing LME and hydrogen embrittlement cracking. As aresult, in a hot stamped body obtained from such a plated steelmaterial, there was still room for improvement relating to the LMEresistance and hydrogen penetration resistance.

Therefore, an object of the present invention is to provide a hotstamped body improved in the LME resistance and hydrogen penetrationresistance and, further, excellent in the corrosion resistance.

Solution to Problem

The present invention to achieve the above object is as follows:

(1) A hot stamped body comprising a steel base material and a platinglayer formed on a surface of the steel base material, wherein theplating layer has a chemical composition comprising, by mass %,

Al: 15.00 to 45.00%,

Mg: 4.50 to 12.00%,

Si: 0.05 to 3.00%,

Ca: 0.05 to 3.00%,

Fe: 20.00 to 50.00%,

Sb: 0 to 0.50%,

Pb: 0 to 0.50%,

Cu: 0 to 1.00%,

Sn: 0 to 1.00%,

Ti: 0 to 1.00%,

Sr: 0 to 0.50%,

Cr: 0 to 1.00%,

Ni: 0 to 1.00%,

Mn: 0 to 1.00%, and

balance: Zn and impurities,

the plating layer comprises an interfacial layer positioned at aninterface with the steel base material and containing Fe and Al and amain layer positioned on the interfacial layer,

the main layer comprises, by area ratio, 10.0 to 85.0% of an Mg—Zncontaining phase and 15.0 to 90.0% of an Fe—Al containing phase,

the Mg—Zn containing phase comprises at least one selected from thegroup consisting of an MgZn phase, Mg₂ Zn₃ phase, and MgZn₂ phase, and

the Fe—Al containing phase comprises at least one of an FeAl phase andFe—Al—Zn phase and an area ratio of the Fe—Al—Zn phase in the main layeris 10.0% or less.

(2) The hot stamped body according to the above (1), wherein thechemical composition of the plating layer comprises, by mass %,

Al: 20.00 to 30.00% and

Mg: 5.50 to 10.00%.

(3) The hot stamped body according to the above (1) or (2), wherein theMg—Zn containing phase comprises an MgZn phase, and an area ratio of theMgZn phase in the main layer is 30.0% or more.

(4) The hot stamped body according to any one of the above (1) to (3),wherein the Mg—Zn containing phase comprises an MgZn phase and Mg₂ Zn₃phase, and an area ratio of a total of the MgZn phase and Mg₂ Zn₃ phasein the main layer is 25.0 to 85.0%.

(5) The hot stamped body according to any one of the above (1) to (4),wherein the Fe—Al containing phase comprises an FeAl phase and an arearatio of the FeAl phase in the main layer is 5.0 to 55.0%.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a hotstamped body improved in the LME resistance and hydrogen penetrationresistance and, further, excellent in the corrosion resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a backscattered electron image (BSE image) of a scanningelectron microscope (SEM) of a plating layer cross-section in aconventional hot stamped body including an Al—Zn—Mg-based plating layer.

FIG. 2 shows a backscattered electron image (BSE image) of a scanningelectron microscope (SEM) of a plating layer cross-section in a hotstamped body according to the present invention.

FIG. 3 shows a backscattered electron image (BSE image) of a scanningelectron microscope (SEM) of a plating layer surface before hot stampingin a hot stamped body according to the present invention.

FIG. 4 is a graph showing a relationship between a point of change of acooling speed when cooling a plating layer and formation of an acicularAl—Zn—Si—Ca phase.

DESCRIPTION OF EMBODIMENTS

<Hot Stamped Body>

The hot stamped body according to an embodiment of the present inventioncomprises a steel base material and a plating layer formed on a surfaceof the steel base material, wherein the plating layer has a chemicalcomposition comprising, by mass %,

Al: 15.00 to 45.00%,

Mg: 4.50 to 12.00%,

Si: 0.05 to 3.00%,

Ca: 0.05 to 3.00%,

Fe: 20.00 to 50.00%,

Sb: 0 to 0.50%,

Pb: 0 to 0.50%,

Cu: 0 to 1.00%,

Sn: 0 to 1.00%,

Ti: 0 to 1.00%,

Sr: 0 to 0.50%,

Cr: 0 to 1.00%,

Ni: 0 to 1.00%,

Mn: 0 to 1.00%, and

balance: Zn and impurities,

the plating layer comprises an interfacial layer positioned at aninterface with the steel base material and containing Fe and Al and amain layer positioned on the interfacial layer,

the main layer comprises, by area ratio, 10.0 to 85.0% of an Mg—Zncontaining phase and 15.0 to 90.0% of an Fe—Al containing phase,

the Mg—Zn containing phase comprises at least one selected from thegroup consisting of an MgZn phase, Mg₂ Zn₃ phase, and MgZn₂ phase, and

the Fe—Al containing phase comprises at least one of an FeAl phase andFe—Al—Zn phase and an area ratio of the Fe—Al—Zn phase in the main layeris 10.0% or less.

For example, if using a conventional Zn-based plated steel material oran Al—Zn-based plated steel material for hot stamping, in general theplated steel material will be heated in the hot stamping to about 900°C. or a higher temperature than that. Zn has a boiling point of about907° C., which is relatively low, therefore at such a high temperature,the Zn in the plating layer will evaporate or melt, resulting in thepartial formation of a high concentration Zn liquid phase in the platinglayer and the penetration of the liquid Zn into the crystal grainboundaries in the steel in some cases causing liquid metal embrittlement(LME) cracking.

On the other hand, in a conventional Al plated steel material notcontaining Zn, LME cracking due to Zn will not occur, but at the time ofheating in the hot stamping, the water vapor in the atmosphere willsometimes be reduced by the Al in the plating layer, resulting in thegeneration of hydrogen. As a result, the generated hydrogen willsometimes penetrate the steel material and cause hydrogen embrittlementcracking. Further, in an Al—Zn-based plated steel material as well,since Zn has a relatively low boiling point as explained above, at thetime of hot stamping at a 900° C. or higher temperature, a part of theZn will evaporate and sometimes will react with the water vapor in theatmosphere and cause the generation of hydrogen. In such a case,hydrogen embrittlement cracking is liable to occur due to hydrogenpenetrating the steel material due to not only the Al, but also the Zn.In addition, from the viewpoint of improvement of the corrosionresistance, regarding the Mg and other elements which are added to theZn-based plated steel material or Al—Zn-based plated steel material,sometime parts thereof will evaporate at the time of heating in hotstamping at a high temperature and, in the same way as the case of Zn,cause production of hydrogen triggering hydrogen embrittlement cracking.

Further, if the elements Zn and/or Mg having the effect of improving thecorrosion resistance at the time of hot stamping at a high temperatureevaporate and parts of those elements are lost, naturally a problem willarise in that it is not possible to maintain sufficient corrosionresistance in the body after hot stamping. Furthermore, if the Zn and/orMg in the plating layer evaporate and are lost, in the plating layerafter the hot stamping, relatively large amounts of Al—Fe-basedintermetallic compounds and/or Zn—Fe-based intermetallic compounds willbe formed between the Fe which had been diffused from the base iron andthe Al and/or Zn in the plating layer. These intermetallic compoundsbecome causes of red rust in corrosive environments.

Therefore, the inventors studied the corrosion resistance, LMEresistance, and hydrogen penetration resistance in hot stamped bodieswhich include Al—Zn—Mg-based plating layers. As a result, the inventorsdiscovered that in a hot stamped body comprising an Al—Zn—Mg-basedplating layer having a predetermined chemical composition and containinga predetermined amount of an Mg—Zn containing phase in the plating layerafter hot stamping, it is possible to remarkably reduce or suppress LMEand penetration of hydrogen into the steel material due to the heatingin the hot stamping and to achieve sufficient corrosion resistance. Inaddition, the inventors discovered that by limiting the amount of theFe—Al—Zn phase contained in the plating layer to within a predeterminedrange, the hydrogen penetration resistance of the hot stamped body isfurther improved. Below, this will be explained more specifically whilereferring to the drawings.

FIG. 1 shows a backscattered electron image (BSE image) of a scanningelectron microscope (SEM) of a plating layer cross-section in aconventional hot stamped body containing an Al—Zn—Mg-based platinglayer. Referring to FIG. 1 , it will be understood that the platinglayer 1 contains a thick oxide layer 2 containing Zn and Mg. The oxidelayer 2 is believed to be the result of at least part of the Zn and Mgevaporating due to heating at about 900° C. in the hot stamping or ahigher temperature than that depositing on the surface of the platinglayer as oxides. On the other hand, a diffusion layer 3 is positionedbelow the plating layer 1. The diffusion layer 3 forms part of the steelbase material 4. The diffusion layer 3 results from the Al constituentin the plating layer diffusing into the steel base material 4 andforming a solid solution due to the heating in the hot stamping.

In a conventional hot stamped body containing an Al—Zn—Mg-based platinglayer such as shown in FIG. 1 , the Zn and Mg evaporate during theheating in the hot stamping, therefore LME and hydrogen penetration intothe steel material occur. Furthermore, the corrosion resistance of thehot stamped body greatly falls due to the loss of at least part of theZn and Mg due to evaporation of these elements and the decrease in theZn and Mg in the metal phase accompanying formation of the oxides. Inaddition, for example, LME cracking is liable to be caused even when theconcentration of Zn in the plating layer 1 relatively rises due toevaporation of Mg.

FIG. 2 shows a backscattered electron image (BSE image) of a scanningelectron microscope (SEM) of a plating layer cross-section in a hotstamped body according to the present invention. Referring to FIG. 2 ,the plating layer 1 comprises an interfacial layer 5 positioned at theinterface with the steel base material 4, more specifically at theinterface with the diffusion layer 3 forming part of the steel basematerial 4, and containing Fe and Al and a main layer 6 positioned onthe interfacial layer 5. The interfacial layer 5, in normal hotstamping, is formed at the interface with the steel base material and ismainly comprised of intermetallic compounds containing Fe and Al. Theinterfacial layer 5 and the diffusion layer 3 positioned beneath it arealmost no different in chemical composition since the metal elements ofthe layers diffuse into each other due to the relatively long heattreatment in the hot stamping for example. Therefore, in the presentinvention, the interfacial layer 5 and the diffusion layer 3 willsometimes not particularly be differentiated and the two together willsometimes be expressed as the “Fe—Al layer 7”.

Further, it will be understood that the main layer 6, in contrast to thecase of FIG. 1 , contains an Mg—Zn containing phase 8 containing atleast one selected from the group consisting of an MgZn phase, Mg₂ Zn₃phase, and MgZn₂ phase, and an Fe—Al containing phase 9 comprising anFeAl phase 9 a. While not shown in FIG. 2 , the Fe—Al containing phase 9sometimes includes, in addition to the FeAl phase 9 a, a relativelysmall amount of an Fe—Al—Zn phase. In particular, it will be understoodthat the main layer 6 shown in FIG. 2 has a structure (island-in-seastructure) of a matrix phase of an Mg—Zn containing phase 8 in whichislands of the Fe—Al containing phase 9 (islands of FeAl phase 9 a andislands of Fe—Al—Zn phase) are present, in particular are presentdispersed. In the hot stamped body according to the present invention,by including an Mg—Zn containing phase 8 such as shown in FIG. 2 in themain layer 6 of the plating layer 1 in a relatively large amount, it ispossible to remarkably reduce or suppress occurrence of LME andpenetration of hydrogen into the steel material and to achievesufficient corrosion resistance. In addition, by controlling the amountof the Fe—Al—Zn phase contained in the main layer 6 to within apredetermined range or by not including an Fe—Al—Zn phase in the mainlayer 6, it is possible to further improve the hydrogen penetrationresistance of the hot stamped body.

While not intending to be bound by any specific theory, in the hotstamped body according to the present invention, as explained in detaillater in relation to the method of production, at the start of heatingin the hot stamping, it is believed that the Ca leached out from theacicular Al—Zn—Si—Ca phase present in the surface structure of theplating layer is preferentially oxidized by the oxygen in the atmosphereand forms a dense Ca-based oxide film at the surface-most part of theplating layer. In other words, it is believed that the acicularAl—Zn—Si—Ca phase present in the surface structure of the plating layerbefore hot stamping functions as a supply source of Ca for forming aCa-based oxide film at the start of heating in hot stamping, then theCa-based oxide film obtained by the oxidation of Ca supplied, morespecifically a Ca- and Mg-containing oxide film, functions as a barrierlayer.

Due to the function of such a barrier layer, it is believed thatevaporation of Zn and Mg in the plating layer to the outside and therelated occurrence of LME and the penetration of hydrogen from theoutside can be decreased or suppressed. As a result, it is believed thatin the body finally obtained after hot stamping, unlike the case of FIG.1 , Zn and Mg can be kept from forming a thick oxide layer 2 in theplating layer 1, can be made present as an Mg—Zn containing phase 8 in arelatively large amount, i.e., in an amount of 10.0 to 85.0% by arearatio in the main layer 6, and therefore the drop in corrosionresistance due to the evaporation of Zn and Mg to the outside can beremarkably suppressed.

Further, the FeAl phase 9 a contained in the Fe—Al containing phase 9,as shown in FIG. 2 , is present in a relatively large amount near theinterface of the main layer 6 and the interfacial layer 5, while theFe—Al—Zn phase (not shown) is present in a relatively large amount nearthe surface of the main layer 6. Therefore, if the content of theFe—Al—Zn phase in the main layer 6 becomes greater, only naturally, theamount of the Fe—Al—Zn phase present near the surface of the main layer6 will also become greater. In such a case, at the time of the heatingin the hot stamping, the water vapor in the atmosphere will be reducedby the Al in the Fe—Al—Zn phase and hydrogen will be generated. As aresult, sometimes the generated hydrogen will penetrate the steelmaterial and cause hydrogen embrittlement cracking. In the hot stampedbody according to the present invention, it is believed that by limitingthe Fe—Al—Zn phase in the main layer 6 to within a predetermined range,i.e, to within an area ratio of 10.0% or less, the amount of hydrogengenerated due to the Fe—Al—Zn phase can be reduced. As a result, it isbelieved that, compared with simply controlling the amount of the Mg—Zncontaining phase in the plating layer, it becomes possible to furtherimprove the hydrogen penetration resistance of the hot stamped body.

Below, the hot stamped body according to an embodiment of the presentinvention will be explained in detail. In the following explanation, the“%” relating to the contents of the constituents means “mass %” unlessotherwise indicated.

[Steel Base Material]

The steel base material according to the embodiment of the presentinvention may be a material having any thickness and composition. It isnot particularly limited, but, for example, is preferably a materialhaving a thickness and composition suitable for application to hotstamping. Such a steel base material is known, and may include, forexample, a steel sheet having a 0.3 to 2.3 mm thickness and comprising,by mass %, C: 0.05 to 0.40%, Si: 0.50% or less, Mn: 0.50 to 2.50%, P:0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.010% orless, and a balance: Fe and impurities (for example, a cold rolled steelsheet), etc. Below, the constituents contained in the steel basematerial preferably applied in the present invention will be explainedin detail.

[C: 0.05 to 0.40%]

Carbon (C) is an element effective for raising the strength of a hotstamped body. However, if the C content is too great, the hot stampedbody will sometimes fall in toughness. Therefore, the C content is 0.05to 0.40%. The C content is preferably 0.10% or more, more preferably0.13% or more. The C content is preferably 0.35% or less.

[Si: 0 to 0.50%]

Silicon (Si) is an element effective for deoxidizing steel. However, ifthe Si content is too great, the Si in the steel diffuses at the time ofheating in the hot stamping and forms oxides at the steel materialsurface. As a result, the efficiency of phosphate treatment sometimesfalls. Further, Si is an element making the Ac₃ point of the steel rise.For this reason, since the heating temperature of the hot stamping hasto be the Ac₃ point or more, if the amount of Si becomes excessive, theheating temperature of the hot stamping of the steel will inevitablybecome higher. In other words, steel with a large amount of Si is heatedto a higher temperature at the time of hot stamping and, as a result,Zn, etc., in the plating layer will unavoidably evaporate. To avoid sucha situation, the Si content is 0.50% or less. The Si content ispreferably 0.30% or less, more preferably 0.20% or less. The Si contentmay also be 0%, but to obtain the effect of deoxidation, etc., the lowerlimit value of the Si content, while changing depending on the desireddeoxidation level, is generally 0.05%.

[Mn: 0.50 to 2.50%]

Manganese (Mn) raises the hardenability and raises the strength of thehot stamped body. On the other hand, even if including Mn in excess, theeffect become saturated. Therefore, the Mn content is 0.50 to 2.50%. TheMn content is preferably 0.60% or more, more preferably 0.70% or more.The Mn content is preferably 2.40% or less, more preferably 2.30% orless.

[P: 0.03% or Less]

Phosphorus (P) is an impurity contained in steel. P segregates at thecrystal grain boundaries to cause a drop in the toughness of the steeland causes a drop in the delayed fracture resistance. Therefore, the Pcontent is 0.03% or less. The P content is preferably as small aspossible and is preferably 0.02% or less. However, excessive reductionof the P content invites a rise in costs, therefore the P content ispreferably 0.0001% or more. The inclusion of P is not essential,therefore the lower limit of the P content is 0%.

[S: 0.010% or Less]

Sulfur (S) is an impurity contained in steel. S forms sulfides to causea drop in the toughness of the steel and cause a drop in the delayedfracture resistance. Therefore, the S content is 0.010% or less. The Scontent is preferably as small as possible and is preferably 0.005% orless. However, excessive reduction of the S content invites a rise incosts, therefore the S content is preferably 0.0001% or more. Theinclusion of S is not essential, therefore the lower limit of the Scontent is 0%.

[sol. Al: 0 to 0.10%]

Aluminum (Al) is effective for deoxidation of steel. However, excessiveinclusion of Al causes the Ac₃ point of the steel material to rise andaccordingly the heating temperature of the hot stamping becomes higherand Zn, etc., in the plating layer unavoidably evaporate. Therefore, theAl content is 0.10% or less, preferably 0.05% or less. The Al contentmay also be 0%, but to obtain the effect of deoxidation, etc., the Alcontent may be 0.01% or more. In this Description, the Al content meansthe content of so-called acid-soluble Al (sol. Al).

[N: 0.010% or Less]

Nitrogen (N) is an impurity unavoidably contained in steel. N formsnitrides to cause a drop in the toughness of the steel. If boron (B) isfurther contained in the steel, N bonds with B to cause a reduction inthe amount of B in solid solution and cause a drop in the hardenability.Therefore, the N content is 0.010% or less. The N content is preferablyas small as possible and is preferably 0.005% or less. However,excessive reduction of the N content invites a rise in costs, thereforethe N content is preferably 0.0001% or more. The inclusion of N is notessential, therefore the lower limit of the N content is 0%.

The basic chemical composition of the steel base material suitable foruse in the embodiment according to the present invention is as explainedabove. Further, the above steel base material may optionally contain oneor more of B: 0 to 0.005%, Ti: 0 to 0.10%, Cr: 0 to 0.50%, Mo: 0 to0.50%, Nb: 0 to 0.10%, and Ni: 0 to 1.00%. Below, these elements will beexplained in detail. The inclusion of these element is not essential,therefore the lower limits of the contents of the elements are 0%.

[B: 0 to 0.005%]

Boron (B) raises the hardenability of steel and raises the strength ofthe steel material after hot stamping, therefore may be included in thesteel base material. However, even if including B in excess, the effectbecomes saturated. Therefore, the B content is 0 to 0.005%. The Bcontent may also be 0.0001% or more.

[Ti: 0 to 0.10%]

Titanium (Ti) can bond with nitrogen (N) to form nitrides and keep thehardenability from dropping due to formation of BN. Further, due to thepinning effect, Ti can refine the austenite grain size and raise thetoughness, etc., of the steel material at the time of heating in hotstamping. However, even if including Ti in excess, the effect becomessaturated. Further, if Ti nitrides precipitate in excess, sometimes thetoughness of the steel will fall. Therefore, the Ti content is 0 to0.10%. The Ti content may be 0.01% or more.

[Cr: 0 to 0.50%]

Chromium (Cr) is effective for raising the hardenability of steel andraising the strength of the hot stamped body. However, if the Cr contentis excessive and a large amount of Cr carbides which are difficult tomelt at the time of heating in hot stamping are formed, it becomesdifficult for the steel to transform to austenite, and conversely thehardenability falls. Therefore, the Cr content is 0 to 0.50%. The Crcontent may also be 0.10% or more.

[Mo: 0 to 0.50%]

Molybdenum (Mo) raises the hardenability of steel. However, even ifincluding Mo in excess, the above effect becomes saturated. Therefore,the Mo content is 0 to 0.50%. The Mo content may also be 0.05% or more.

[Nb: 0 to 0.10%]

Niobium (Nb) is an element which forms carbides to refine the crystalgrains at the time of hot stamping and raise the toughness of the steel.However, if including Nb in excess, the above effect becomes saturatedand further the hardenability falls. Therefore, the Nb content is 0 to0.10%. The Nb content may also be 0.02% or more.

[Ni: 0 to 1.00%]

Nickel (Ni) is an element able to suppress embrittlement caused bymolten Zn at the time of the heating in the hot stamping. However, evenif including Ni in excess, the effect becomes saturated. Therefore, theNi content is 0 to 1.00%. The Ni content may also 0.10% or more.

In the steel base material according to the embodiment of the presentinvention, the balance other than the above constituents is comprised ofFe and impurities. The “impurities” in the steel base material meanconstituents entering due to various factors in the production process,first and foremost the raw materials such as the ore and scrap, whenindustrially producing the hot stamped body according to the embodimentof the present invention, and not intentionally added to the hot stampedbody.

[Plating Layer]

According to the embodiment of the present invention, a plating layer isformed on the surface of the above steel base material. For example, ifthe steel base material is a steel sheet, the plating layer is formed onat least one surface of the steel sheet, i.e., one surface or bothsurfaces of the steel sheet. The plating layer comprises an interfaciallayer positioned at the interface with the steel base material andcontaining Fe and Al and a main layer positioned on the interfaciallayer. The plating layer has the following average composition.

[Al: 15.00 to 45.00%]

Al is an element essential for suppressing the evaporation of the Zn andMg at the time of the heating in the hot stamping. As explained above,it is believed that due to the presence of the acicular Al—Zn—Si—Caphase in the surface structure of the plating layer before the hotstamping, the Ca leaching out from the acicular Al—Zn—Si—Ca phase at thestart of the heating in the hot stamping is preferentially oxidized bythe oxygen in the atmosphere and a dense Ca-based oxide film, morespecifically a Ca- and Mg-containing oxide film, is formed on theoutermost surface of the plating layer. Such a Ca-based oxide film isbelieved to function as a barrier layer for suppressing evaporation ofthe Zn and Mg. To express the function of the barrier layer, the contentof Al in the plating layer after hot stamping has to be 15.00% or more,preferably is 20.00% or more or 25.00% or more. On the other hand, ifthe Al content is more than 45.00%, Al₄ Ca and other intermetalliccompounds are preferentially formed at the plating layer before the hotstamping and formation of the acicular Al—Zn—Si—Ca phase in a sufficientamount becomes difficult. Therefore, the Al content is 45.00% or less,preferably 40.00% or less or 35.00% or less.

[Mg: 4.50 to 12.00%]

Mg is an element effective for improving the corrosion resistance of theplating layer and improving the coating blistering, etc. Further, Mg hasthe effect of forming liquid phase Zn—Mg and suppressing LME cracking atthe time of heating in the hot stamping. If the Mg content is low, thepossibility of LME occurring increases. From the viewpoint ofimprovement of the corrosion resistance and suppression of the LME, theMg content is 4.50% or more, preferably 5.00% or more or 5.50% or more.On the other hand, if the Mg content is too high, an excessivesacrificial corrosion action tends to cause coating blistering and flowrust to rapidly become larger. Therefore, the Mg content is 12.00% orless, preferably 10.00% or less.

[Si: 0.05 to 3.00%]

Si is an element essential for suppressing evaporation of Zn and Mg atthe time of the heating in the hot stamping. As explained above, due tothe presence of the acicular Al—Zn—Si—Ca phase in the surface structureof the plating layer before the hot stamping, it is possible to form abarrier layer comprised of a Ca-based oxide film for suppressingevaporation of Zn and Mg at the time of heating in the hot stamping. Toexpress the function of the barrier layer, the Si content in the platinglayer after hot stamping has to be 0.05% or more, preferably is 0.10% ormore or 0.40% or more. On the other hand, if the Si content isexcessive, an Mg₂ Si phase is formed at the interface of the steel basematerial and the plating layer at the plating layer before hot stampingand the corrosion resistance greatly deteriorates. Further, if the Sicontent is excessive, the Mg₂ Si phase is preferentially formed at theplating layer before the hot stamping and it becomes difficult to makethe acicular Al—Zn—Si—Ca phase form in a sufficient amount. Therefore,the Si content is 3.00% or less, preferably 1.60% or less, morepreferably 1.00% or less.

[Ca: 0.05 to 3.00%]

Ca is an element essential for suppressing evaporation of Zn and Mg atthe time of heating in the hot stamping. As explained above, due to thepresence of the acicular Al—Zn—Si—Ca phase in the surface structure ofthe plating layer before the hot stamping, it is possible to form abarrier layer comprised of a Ca-based oxide film for suppressingevaporation of Zn and Mg at the time of the heating in the hot stamping.To express the function of the barrier layer, the Ca content in theplating layer after hot stamping has to be 0.05% or more, preferably is0.40% or more. On the other hand, if the Ca content is excessive, Al₄ Caand other intermetallic compounds are preferentially formed at theplating layer before the hot stamping and it becomes difficult to makethe acicular Al—Zn—Si—Ca phase form in a sufficient amount. Therefore,the Ca content is 3.00% or less, preferably 2.00% or less, morepreferably 1.50% or less.

[Fe: 20.00 to 50.00%]

If heating the plated steel material at the time of hot stamping, the Fefrom the steel base material diffuses in the plating layer, thereforethe plating layer inevitably contains Fe. Fe bonds with the Al in theplating layer to form at the interface with the steel base material aninterfacial layer mainly comprised of an intermetallic compoundcontaining Fe and Al and further form an Fe—Al containing phase in themain layer positioned on the interfacial layer. If the Fe content islow, the amount of the Fe—Al containing phase decreases, therefore thestructure of the main layer easily collapses. More specifically, if theFe content is low, the Zn and Mg contents relatively increase, thereforeat the time of the heating in the hot stamping, these elements easilyevaporate and as a result hydrogen penetration easily occurs. Therefore,the Fe content is 20.00% or more, preferably 25.00% or more. On theother hand, if the Fe content is too high, the amount of the Fe—Alcontaining phase in the main layer becomes greater and the amount of theMg—Zn containing phase in the main layer relatively decreases, thereforethe corrosion resistance falls. Therefore, the Fe content is 50.00% orless, preferably 45.00% or less, more preferably 40.00% or less.

The chemical composition of the plating layer is as explained above.Furthermore, the plating layer may optionally contain one or more of Sb:0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, Ti: 0 to1.00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to1.00%. While not particularly limited to this, from the viewpoint ofcausing the actions and functions of the above basic constituentsforming the plating layer to be sufficiently manifested, the totalcontent of these elements is preferably 5.00% or less, more preferably2.00% or less. Below, these elements will be explained in detail.

[Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, and Ti:0 to 1.00%]

Sb, Pb, Cu, Sn, and Ti can be contained in the Mg—Zn containing phasepresent in the main layer, but if within predetermined ranges ofcontents, do not detrimentally affect the performance of the hot stampedbody. However, if the contents of the elements are excessive, at thetime of the heating in the hot stamping, sometimes oxides of theseelements will precipitate and cause deterioration of the surfaceproperties of the hot stamped body and the phosphate treatment willbecome poor and the corrosion resistance after coating will deteriorate.Furthermore, if the Pb and Sn contents become excessive, the LMEresistance will tend to fall. Therefore, the contents of Sb and Pb are0.50% or less, preferably 0.20% or less, while the contents of Cu, Sn,and Ti are 1.00% or less, preferably 0.80% or less, more preferably0.50% or less. On the other hand, the contents of elements may also be0.01% or more. These elements are not essential. The lower limits of thecontents of these elements are 0%.

[Sr: 0 to 0.50%]

Sr can be included in the plating bath at the time of production of theplating layer so as to suppress the formation of the top dross formed onthe plating bath. Further, Sr tends to suppress oxidation by air at thetime of heating in hot stamping, therefore can suppress color changes inthe body after hot stamping. These effects are exhibited even in smallamounts, therefore the Sr content may be 0.01% or more. On the otherhand, if the Sr content is excessive, the occurrence of coatingblistering and flow rust becomes larger and the corrosion resistancetends to deteriorate. Therefore, the Sr content is 0.50% or less,preferably 0.30% or less, more preferably 0.10% or less.

[Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%]

Cr, Ni, and Mn concentrate near the interface of the plating layer andthe steel base material and have the effect of eliminating spangles ofthe plating layer surface, etc. To obtain such an effect, the contentsof Cr, Ni, and Mn are preferably respectively 0.01% or more. On theother hand, these elements may be included in the interfacial layer orincluded in the Fe—Al containing phase present in the main layer.However, if the contents of these elements are excessive, the coatingblistering and flow rust become greater and the corrosion resistancetends to deteriorate. Therefore, the contents of Cr, Ni, and Mn arerespectively 1.00% or less, preferably 0.50% or less, more preferably0.10% or less.

[Balance: Zn and Impurities]

The balance in the plating layer aside from the above constituents iscomprised of Zn and impurities. Zn is an essential constituent in theplating layer from the viewpoint of preventing corrosion. Zn is presentmainly as the Mg—Zn containing phase in the main layer of the platinglayer and greatly contributes to improvement of the corrosionresistance. If the Zn content is less than 3.00%, sometimes a sufficientcorrosion resistance cannot be maintained. Therefore, the Zn content ispreferably 3.00% or more. The lower limit of the Zn content may be10.00%, 15.00%, or 20.00%. On the other hand, if the Zn content is toohigh, at the time of the heating in the hot stamping, Zn easilyevaporates and as a result LME and hydrogen penetration easily occur.Therefore, the Zn content is preferably 50.00% or less. The upper limitof the Zn content may be 45.00%, 40.00%, or 35.00%. Further, Zn can besubstituted by Al, therefore a small amount of Zn can form a solidsolution with the Fe in the Fe—Al containing phase. Further, the“impurities” in the plating layer mean constituents entering due tovarious factors in the production process, first and foremost the rawmaterials, when producing the plating layer, and not intentionally addedto the plating layer. In the plating layer, the impurities may containelements other than the elements explained above in trace amounts to anextent not detracting from the effect of the present invention.

The chemical composition of the plating layer is determined bydissolving the plating layer in an acid solution to which an inhibitoris added for inhibiting corrosion of the steel base material andmeasuring the obtained solution by the ICP (high frequency inductivelycoupled plasma) emission spectrometry method. In this case, the measuredchemical composition is the average composition of the total of the mainlayer and the interfacial layer.

The thickness of the plating layer may be, for example, 3 to 50 μm.Further, if the steel base material is a steel sheet, the plating layermay be provided at both surfaces of the steel sheet or may be providedat only one surface. The amount of deposition of the plating layer isnot particularly limited, but for example may be 10 to 170 g/m² persurface. The lower limit may be 20 or 30 g/m² and the upper limit may be150 or 130 g/m². In the present invention, the amount of deposition ofthe plating layer is determined from the change in weight before andafter acid washing by dissolving the plating layer in an acidic solutionto which an inhibitor for inhibiting corrosion of the base iron has beenadded.

[Interfacial Layer]

The interfacial layer is a layer containing Fe and Al, more specificallya layer at which, at the time of the heating in the hot stamping, the Fefrom the steel base material diffuses in the plating layer and bondswith the Al in the plating layer and is mainly comprised of anintermetallic compound containing Fe and Al.

[Main Layer]

The main layer includes an area ratio of 10.0 to 85.0% of an Mg—Zncontaining phase and 15.0 to 90.0% of an Fe—Al containing phase. Themain layer has the effect of inhibiting the formation of scale at thetime of hot stamping and contributes to corrosion resistance of the hotstamped body as well. The main layer has a mixed structure of an Mg—Zncontaining phase and Fe—Al containing phase and generally, as shown inFIG. 2 , has the structure (island-in-sea structure) of a matrix phaseof an Mg—Zn containing phase 8 in which islands of Fe—Al containingphase 9 are present, in particular are present dispersed. If referringto FIG. 2 , the islands of the Fe—Al containing phase 9 include not onlythe islands of the FeAl phase 9 a, but also groups of islands of theFeAl phase 9 a, etc., adjoining each other.

[Mg—Zn Containing Phase]

In an embodiment according to the present invention, by configuring theplating layer after hot stamping so that Zn and Mg having a corrosionresistance improving effect are present as an Mg—Zn containing phase inthe main layer in an area ratio of an amount of 10.0 to 85.0%,occurrence of LME and hydrogen penetration to the steel material due tothe heating at the time of hot stamping can be remarkably reduced orsuppressed and, even in the body after hot stamping, sufficientcorrosion resistance can be achieved. If the area ratio of the Mg—Zncontaining phase is less than 10.0%, it is not possible to sufficientlyobtain such an effect. Therefore, the area ratio of the Mg—Zn containingphase is 10.0% or more, preferably 15.0% or more, more preferably 25.0%or more. On the other hand, the area ratio of the Mg—Zn containing phasemay be 85.0% or less, for example, may be 80.0% or less, 75.0% or less,or 70.0% or less.

The Mg—Zn containing phase includes at least one phase selected from thegroup consisting of an MgZn phase, Mg₂ Zn₃ phase, and MgZn₂ phase. Here,the MgZn phase, Mg₂ Zn₃ phase, and MgZn₂ phase are intermetalliccompounds, therefore while the atomic ratios of Mg and Zn of the phasesmay be considered to be substantially constant, in actuality theyfluctuate somewhat since sometimes Al, Fe, etc., dissolve partially.Therefore, in the present invention, in phases having a chemicalcomposition in which the total of the Mg and Zn contents is 90.0% ormore, a phase where the atomic ratio of Mg/Zn is 0.90 to 1.10 is definedas an MgZn phase, a phase where an atomic ratio of Mg/Zn is 0.58 to 0.74is defined as an Mg₂Zn₃ phase, and a phase where an atomic ratio ofMg/Zn is 0.43 to 0.57 is defined as an MgZn₂ phase. By the Mg—Zncontaining phase including these phases, the corrosion resistance of thehot stamped body can be remarkably improved. In particular, when theMg—Zn containing phase includes an MgZn phase and/or Mg₂ Zn₃ phase, itis possible to suppress LME at the time of hot stamping. To reliablyobtain such an effect, the Mg—Zn containing phase preferably includes anMgZn phase with a large Mg content. The area ratio of the MgZn phase inthe main layer is preferably 5.0% or more and 10.0% or more is morepreferable. 30.0% or more or 40.0% or more is also possible. Further,the Mg—Zn containing phase preferably includes an MgZn phase and Mg₂ Zn₃phase. The area ratio of the total of the MgZn phase and Mg₂ Zn₃ phasein the main layer is preferably 10.0% or more or 25.0% or more. 40.0% ormore or 50.0% or more is also possible. On the other hand, it may be85.0% or less, 80.0% or less, 75.0% or less, or 70.0% or less. Bycontrolling the Mg—Zn containing phase to within such a range, it ispossible to remarkably reduce or suppress the occurrence of LME andhydrogen penetration to the steel material occurring due to the heatingat the time of hot stamping and possible, even in the body after hotstamping, to achieve sufficient corrosion resistance.

[Fe—Al Containing Phase]

As explained above, the main layer includes an area ratio of 15.0 to90.0% of an Fe—Al containing phase. If the area ratio of the Fe—Alcontaining phase is more than 90.0%, the amount of the Mg—Zn containingphase contained in the main layer becomes smaller and the corrosionresistance falls. On the other hand, the area ratio of the Fe—Alcontaining phase may be 15.0% or more, for example, may be 20.0% or moreor 25.0% or more. The Fe—Al containing phase becomes a barrier at thetime corrosion progresses in the Mg—Zn containing phase, therefore byestablishing the presence of the Fe—Al containing phase, the corrosionresistance can be improved. Explaining this in more detail, the Fe—Alcontaining phase (Fe—Al—Zn phase and FeAl phase) is present in the mainlayer not as a laminar structure, but as island structures, therefore ifcorrosion progresses in the Mg—Zn containing phase having the corrosionresistance improving effect, the corrosion will proceed in a spottedstate avoiding these islands of the Fe—Al containing phase. As a result,it is believed possible to delay progress of corrosion of the Mg—Zncontaining phase.

The Fe—Al containing phase includes the Fe—Al—Zn phase and FeAl phase.The area ratio of the Fe—Al—Zn phase in the main layer is more than10.0% or less. In the present invention, the Fe—Al containing phasemeans a phase having a chemical composition where the total of Fe, Al,and Zn is 90.0% or more. In the Fe—Al containing phase having such achemical composition, a phase where the Zn content is 1.0% or more isdefined as an Fe—Al—Zn phase and a phase where the Zn content is lessthan 1.0% is defined as an FeAl phase. While not intending to be boundby any specific theory, it is believed that the Fe—Al—Zn phase and FeAlphase do not grow at the interface of the plating layer and the steelbase material from the steel base material to inside the plating layerin a layer shape, but form nuclei of spherical shapes in the platinglayer in the molten state at the time of the heating in the hot stampingand then grow into island shapes.

As explained in detail later, by suitably controlling the productionconditions of the plated steel material before hot stamping, it ispossible to establish the presence of the acicular Al—Zn—Si—Ca phasedispersed in the surface structure of the plating layer. As a result, itis possible to suppress the evaporation of Zn and Mg at the time of theheating in the hot stamping. By suppressing the evaporation of Zn andMg, it is believed that nuclei are formed inside the main layer in themolten state and the Fe—Al containing phase grows to island shapes. Asexplained above, the Fe—Al containing phase, in particular the Fe—Al—Znphase and FeAl phase, has island shapes. While not particularly limited,the aspect ratio almost never is more than 5.0. In general, the Fe—Alcontaining phase has island shapes of an aspect ratio of 5.0 or less,for example, 4.0 or less or 3.0 or less. The lower limit of the aspectratio is not particularly prescribed, but, for example, may be 1.0 ormore, 1.2 or more, or 1.5 or more. In the present invention, the “aspectratio” means the ratio of the longest axis of the Fe—Al containing phase(Fe—Al—Zn phase and FeAl phase) (long axis) and the longest axis in theaxes of the Fe—Al containing phase perpendicular to the same (shortaxis).

As explained above, the FeAl phase contained in the Fe—Al containingphase is present in a relatively large amount near the interface of theplating layer and Fe—Al layer, while the Fe—Al—Zn phase is present in arelatively large amount near the surface of the plating layer. In such acase, at the time of the heating in the hot stamping, water vapor in theatmosphere is reduced by the Al in the Fe—Al—Zn phase and hydrogen isgenerated. Therefore, in an embodiment according to the presentinvention, by limiting the Fe—Al—Zn phase in the main layer to an arearatio of 10.0% or less in range, it is possible to reduce the amount ofhydrogen generated due to the Fe—Al—Zn phase. As a result, compared withwhen just controlling the amount of the Mg—Zn containing phase in themain layer, it becomes possible to further improve the hydrogenpenetration resistance of the hot stamped body.

Further, by suitably adjusting the heat treatment in the hot stamping,it is possible to control the contents of the Fe—Al—Zn phase and FeAlphase in the main layer. In an embodiment according to the presentinvention, the area ratio of the Fe—Al—Zn phase in the main layer ispreferably 8.0% or less, more preferably 5.0% or less, most preferably3.0% or less and may also be 0%. Further, in an embodiment according tothe present invention, the area ratio of the FeAl phase in the mainlayer may for example, be 5.0% or more, 10.0% or more, 15.0% or more, or20.0% or more and may be 80.0% or less, 70.0% or less, 55.0% or less, or40.0% or less.

[Other Intermetallic Compounds]

The main layer may contain other intermetallic compounds besides thosecontained in the Mg—Zn containing phase and Fe—Al containing phase. Theother intermetallic compounds are not particularly limited, but, forexample, intermetallic compounds containing Si and Ca or other elementscontained in the plating layer, specifically Mg₂ Si, Al₄ Ca, etc., maybe mentioned. However, if the area ratio of the other intermetalliccompounds in the main layer becomes too large, sometimes it is notpossible to sufficiently secure the Mg—Zn containing phase and/or Fe—Alcontaining phase. Therefore, the area ratio of the other intermetalliccompounds, for example, the area ratio of the Mg₂ Si and Al₄ Ca, ispreferably a total of 10.0% or less. 5.0% or less is more preferable.

[Oxide Layer]

The surface of the plating layer is sometimes formed with an oxide layerdue to oxidation of the plating constituents. Such an oxide layer isliable to cause a drop in the chemical convertibility andelectrodeposition coatability after hot stamping. Therefore, thethickness of the oxide layer is preferably small. For example, it ispreferably 1.0 μm or less. If the Zn and Mg evaporate at the time of hotstamping, a thick Mg—Zn containing oxide layer of more than 1.0 μm isformed.

[Fe—Al Layer]

In an embodiment according to the present invention, as shown in FIG. 2, sometimes an Fe—Al layer 7 is formed beneath the main layer 6. TheFe—Al layer contains mainly Fe and Al. More specifically, it is believedthat the Fe—Al layer is formed by further diffusion of the metalelements of the above-explained interfacial layer and the diffusionlayer positioned beneath the interfacial layer due for example to therelatively long heat treatment in the hot stamping. If the Fe—Al layerbecomes too thick, the Al constituent in the plating layer, inparticular the main layer, becomes too small, so this is not preferable.Therefore, the thickness of the Fe—Al layer is generally 25.0 μm orless, preferably 20.0 μm or less, more preferably 15.0 μm or less, mostpreferably 10.0 μm or less.

The thicknesses of the plating layer, the Fe—Al layer, and the oxidelayer are determined by cutting out a test piece from the hot stampedbody, burying it in a resin, etc., then polishing the cross-section andmeasuring the image observed by an SEM. Further, if examining these in abackscattered electron image of the SEM, the contrast at the time ofobservation will differ depending on the metal constituents, thereforeit is possible to identify the layers and confirm the thicknesses of thelayers. The thicknesses of the plating layer, the Fe—Al layer, and theoxide layer are determined by performing similar observation in three ormore different fields and finding the averages of these.

In the present invention, the area ratios of the phases of the mainlayer are determined in the following way. First, a prepared sample iscut into a 25 mm×15 mm size, and any cross-section of the plating layeris photographed by a power of 1500× by a scanning electron microscope(SEM). From the BSE image of the same and an SEM-EDS mapping image, thearea ratios of the phases at the main layer were measured by computerimage processing. The averages of the measurement values at any fivefields (however, the measured areas in the fields are 400 μm² or more)were determined as the area ratios of the MgZn phase, Mg₂ Zn₃ phase,MgZn₂ phase, FeAl phase, Fe—Al—Zn phase, and other intermetalliccompounds. Further, the area ratio of the Mg—Zn containing phase wasdetermined as the area ratio of the total of the MgZn phase, Mg₂ Zn₃phase, and MgZn₂ phase. Similarly, the area ratio of the Fe—Alcontaining phase was determined as the area ratio of the total of theFeAl phase and Fe—Al—Zn phase.

<Method for Producing Hot Stamped Body>

Next, a preferred method for producing the hot stamped body according tothe embodiment of the present invention will be explained. The followingexplanation is intended to illustrate a characteristic method forproducing a hot stamped body according to the embodiment of the presentinvention and is not intended to limit the hot stamped body to oneproduced by a production method as explained below.

The above production method comprises forming the steel base material,forming a plating layer on the steel base material, and hot stamping(hot pressing) the steel base material on which the plating layer isformed. Below, each step will be explained in detail.

[Step of Forming Steel Base Material]

In the step of forming the steel base material, for example, first,molten steel having the same chemical composition as that explained forthe steel base material is produced. The produced molten steel is usedto produce a slab by a casting method. Alternatively, the producedmolten steel may be used to produce an ingot by the ingot making method.Next, the slab or ingot is hot rolled to produce the steel base material(hot rolled steel sheet). In accordance with need, the hot rolled steelsheet may be pickled, then the hot rolled steel sheet may be coldrolled. The obtained cold rolled steel sheet may be used as the steelbase material.

[Step of Forming Plating Layer]

Next, in the step of forming the plating layer, a plating layer havingthe predetermined chemical composition is formed on at least one surfaceof the steel base material, preferably on both surfaces.

More specifically, first, the above steel base material is reduced byheating in an N₂—H₂ mixed gas atmosphere at a predetermined temperatureand time, for example, a temperature of 750 to 850° C., then is cooledin a nitrogen atmosphere or other inert atmosphere until near theplating bath temperature. Next, the steel base material is dipped in aplating bath having a predetermined chemical composition for 0.1 to 60seconds, then is pulled up and adjusted in amount of deposition of theplating layer to within a predetermined range by immediately blowing N₂gas or air by the gas wiping method.

Further, the amount of deposition of the plating layer is preferably 10to 170 g/m² per surface. In the present step, as an aid to platingdeposition, it is also possible to apply Ni preplating, Sn preplating,or other preplating. However, these preplatings cause changes in thealloying reactions, therefore the amount of deposition of the preplatingis preferably 2.0 g/m² per surface or less.

Finally, the steel base material on which the plating layer is depositedis cooled, whereby the plating layer is formed on one surface or bothsurfaces of the steel base material. In the present method, at the timeof this cooling, it is important to form the acicular Al—Zn—Si—Ca phaseof the intermetallic compound comprised of mainly Al, Zn, Si, and Ca inthe surface structure of the plating layer. FIG. 3 shows a backscatteredelectron image (BSE image) of a scanning electron microscope (SEM) ofthe plating layer surface before hot stamping of the hot stamped bodyaccording to the present invention. Referring to FIG. 3 , it will beunderstood that in the surface structure of the plating layer, inaddition to an α phase 11 (dendrite structure in FIG. 3 ) and α/τeutectic phase 12, the acicular Al—Zn—Si—Ca phase 13 is present in arelatively large amount. The α phase is a structure mainly comprised ofAl and Zn, while the τ phase is a structure mainly comprised of Mg, Zn,and Al.

While not intending to be bound by any specific theory, it is believedthat the acicular Al—Zn—Si—Ca phase 13 shown in FIG. 3 functions as thesupply source of Ca for forming a Ca-based oxide film at the start ofheating in the hot stamping. More specifically, it is believed that dueto the presence of the acicular Al—Zn—Si—Ca phase 13 in the surfacestructure of the plating layer before the hot stamping, the Ca leachingout from the acicular Al—Zn—Si—Ca phase 13 at the start of the heatingin the hot stamping is preferentially oxidized by the oxygen in theatmosphere and forms a dense Ca-based oxide film, more specifically aCa- and Mg-containing oxide film, at the surface-most part of theplating layer. It is believed that such a Ca-based oxide film functionsas a barrier layer for suppressing evaporation of Zn and Mg. Inparticular, by the acicular Al—Zn—Si—Ca phase 13 being present in apredetermined amount, more specifically an area ratio of 2.0% or more,in the surface structure of the plating layer, such a function as abarrier layer is effectively exhibited. Therefore, it is possible toreduce or suppress the evaporation of Zn and Mg in the plating layer tothe outside and penetration of hydrogen from the outside at the time ofhot stamping and further possible to remarkably suppress the drop incorrosion resistance due to evaporation of Zn and Mg to the outside.

In the present method, suitably controlling the cooling conditions atthe time of solidification of the plating layer in the liquid phasestate, more specifically cooling the steel base material on which theplating layer is deposited in two stages, is extremely important for theacicular Al—Zn—Si—Ca phase to be formed in a predetermined amount in thesurface structure of the plating layer. Explained in more detail, thespecific value of the cooling speed can change in accordance with thechemical composition, etc., of the plating layer, but to make theacicular Al—Zn—Si—Ca phase be reliably formed in a predetermined amount,it is effective to first cool the steel base material on which theplating layer is deposited by a 14° C./s or more, preferably 15° C./s ormore, average cooling speed from the bath temperature (in general, 500to 700° C.) down to 450° C., then cool it by a 5.5° C./s or less,preferably 5° C./s or less, average cooling speed from 450° C. to 350°C. By such cooling conditions, i.e., by two-stage cooling of fastcooling and slow cooling, at the time of the first fast cooling, asupersaturated state is created to produce a state in which nuclei ofthe acicular Al—Zn—Si—Ca phase can easily form and a large amount ofnuclei is formed and, at the time of the next slow cooling, the nucleiis made to slowly grow, whereby an area ratio of 2.0% or more of theacicular Al—Zn—Si—Ca phase is formed in the surface structure of theplating layer, in particular is formed dispersed. As a result, even inthe case of a heating temperature of 900° C. or more in hot stamping, itbecomes possible to suppress evaporation of Zn and Mg and is possible toremarkably reduce or suppress LME and hydrogen penetration into thesteel material and achieve sufficient corrosion resistance even in abody after hot stamping. On the other hand, if not performing the abovetwo-stage cooling, it is not possible to form the acicular Al—Zn—Si—Caphase in the surface structure of the plating layer or not possible toform it in a sufficient amount, therefore at the time of the heating inthe hot stamping, much of the Zn and Mg in the plating layer evaporates.Part of the evaporated Zn and Mg is deposited as oxides on the steelbase material. In general, a thick Mg—Zn containing oxide layer of morethan 1.0 μm, for example, 2.0 μm or more or 3.0 μm or more, is formed.As a result, the LME resistance, hydrogen penetration resistance, andcorrosion resistance of the obtained hot stamped body greatly fall.Further, if an acicular Al—Zn—Si—Ca phase is formed, but the amountformed is not necessarily sufficient, sometimes the desired area ratioof the Mg—Zn containing phase or Fe—Al containing phase cannot beachieved.

If the point of change of the cooling speed of the fast cooling and slowcooling becomes higher than about 450° C., sometimes nuclei of theacicular Al—Zn—Si—Ca phase are not sufficiently formed. On the otherhand, if the point of change of the cooling speed becomes lower thanabout 450° C., sometimes the nuclei formed cannot be made tosufficiently grow. Whatever the case, it becomes difficult to render theacicular Al—Zn—Si—Ca phase present in a predetermined amount, morespecifically an area ratio of 2.0% or more in amount in the surfacestructure of the plating layer. Therefore, the point of change of thecooling speed, as explained later, has to be selected from 425 to 475°C. in range. To reliably form 2.0% or more of the acicular Al—Zn—Si—Caphase, as explained above, 450° C. is preferable.

[Step of Hot Stamping (Hot Pressing)]

Finally, in the step of hot stamping (hot pressing), the steel basematerial provided with the plating layer is hot pressed. The presentstep is performed by loading the steel base material provided with theplating layer in a heating furnace, holding it for a predeterminedholding time after reaching 900° C., then hot pressing it. The above“holding time” means the holding time from 900° C. or more to less than1000° C. after reaching 900° C. The specific value of the holding timecan change according to the holding temperature and the chemicalcomposition of the plating layer, etc., but in general is more than 4minutes. To reliably obtain the hot stamped body according to theembodiment of the present invention having the plating layer providedwith the main layer including the above explained Mg—Zn containing phaseand Fe—Al containing phase, the time is 4.5 minutes or more and 6minutes or less or 7 minutes or less.

EXAMPLES

Below, examples will be used to explain the present invention in moredetail, but the present invention is not limited to these examples inany way.

Example A

In the present example, various hot stamped bodies according toembodiments of the present invention were produced under variousconditions and were investigated for characteristics.

First, molten steel comprising, by mass %, a C content of 0.20%, Sicontent of 0.20%, Mn content of 1.30%, P content of 0.01%, S content of0.005%, sol. Al content of 0.02%, N content of 0.002%, B content of0.002%, Ti content of 0.02%, Cr content of 0.20%, and balance of Fe andimpurities was used to produce a slab by continuous casting. Next, theslab was hot rolled to produce hot rolled steel sheet, the hot rolledsteel sheet was pickled, then the sheet was cold rolled to produce acold-rolled steel sheet (steel base material) having a 1.4 mm sheetthickness.

Next, the produced steel base material was cut to 100 mm×200 mm, thenthe steel base material was plated using a batch type hot dip coatingapparatus made by Rhesca. More specifically, first, the produced steelbase material was reduced by heating in a furnace with an oxygenconcentration of 20 ppm or less in an N₂-5% H₂ mixed gas atmosphere at800° C., then was cooled in N₂ down to the plating bath temperature+20°C. Next, the steel base material was dipped in a plating bath having apredetermined chemical composition for about 3 seconds, then was pulledup by a pull-up speed of 20 to 200 mm/s and adjusted by N₂ gas wiping toan amount of deposition of the plating layer of the value shown inTable 1. Next, the steel base material on which the plating layer wasdeposited was cooled in two stages under the conditions shown in Table1, whereby a plated steel material on the two surfaces of which aplating layer was formed was obtained. The sheet temperature wasmeasured using a thermocouple spot welded to the center part of thesteel base material.

Next, the obtained plated steel material was hot stamped. Specifically,the hot stamping was performed by loading the plated steel material intoa heating furnace, then heating it to a temperature of 900° C. andholding it there for a predetermined time, then hot pressing it by a dieequipped with a water cooling jacket. As the heat treatment conditionsat the time of hot stamping (HS), either of the following conditions Xand Y was selected. The quenching by the die was controlled to give acooling speed of 50° C./s or more up to about the martensitetransformation start point (410° C.).

X: Holding at 900° C. for 4.5 minutes

Y: Holding at 900° C. for 6 minutes

[Table 1]

TABLE 1 Method of production Bath Amount temper- of ature 450 todeposition cooling 350° C. of plating Bath speed speed HS Chemicalcomposition of plating layer (mass%) layer per temper- to 450° C.average heat Others surface ature average cooling treat- No. Class Zn AlMg Si Ca Fe Type Total value (g/m²) (° C.) (° C./s) (° C./s) ment 1Comp. ex.  8.25 15.10   0.10 0.03 0.02 76.50 — 0.00 0.5 520 15.0 5.0 X 2Comp. ex. 10.63 13.10   0.20 0.04 0.01 76.00 Pb:0.02 0.02 2.1 520 15.05.0 X 3 Example 42.48 15.00  6.40 0.32 0.80 35.00 — 0.00 50.4 530 15.05.0 X 4 Comp. ex. 26.79 20.60 15.50 0.32 0.79 36.00 — 0.00 50.4 570 15.05.0 X 5 Comp. ex.  9.51 14.10   0.30 0.05 0.04 76.00 — 0.00 1.8 530 15.05.0 X 6 Example 41.74 18.50  4.50 0.30 0.76 34.00 Ni:0.20 0.20 50.4 58015.0 5.0 X 7 Example 40.05 18.50  5.10 0.30 0.55 35.50 — 0.00 41.2 58015.0 5.0 X 8 Example 34.34 22.50  5.40 0.30 0.66 36.50 Mn:0.30 0.30 21.5580 15.0 5.0 X 9 Example 31.64 23.00  6.50 0.30 0.76 37.80 — 0.00 120.0580 15.0 5.0 Y 10 Example 31.88 23.00  5.90 0.37 0.74 38.10 Sr:0.01 0.0118.0 580 15.0 5.0 X 11 Example 33.16 23.10  5.90 0.30 0.44 37.10 — 0.0025.5 580 15.0 5.0 X 12 Example 30.62 23.50  6.10 0.40 0.88 38.50 — 0.0036.0 580 15.0 5.0 X 13 Example 28.65 23.50  6.20 0.41 0.07 41.10 Sb:0.070.07 21.0 580 15.0 5.0 X 14 Example 27.35 23.47  6.20 0.42 0.76 41.00Ni:0.80 0.80 43.2 580 15.0 5.0 Y 15 Comp. ex. 42.30 25.10 11.50 0.400.70 20.00 — 0.00 41.2 620   9.0 5.0 X 16 Comp. ex.  1.74 22.00   0.200.05 0.01 76.00 — 0.00 1.1 580 15.0 15.0   X 17 Comp. ex.  2.05 22.00  0.10 0.04 0.01 75.80 — 0.00 1.3 580   5.0 5.0 X 18 Comp. ex.  2.44 22.05 0.15 0.05 0.01 75.30 — 0.00 1.0 580 15.0 7.0 X 19 Comp. ex. 30.30 22.10 8.30 3.10 0.10 36.10 — 0.00 20.6 580 15.0 5.0 X 20 Comp. ex. 28.8020.50  8.30 0.40 3.20 38.80 — 0.00 20.8 580 15.0 5.0 X 21 Example 21.0225.60  6.10 0.30 0.88 46.10 — 0.00 41.1 580 15.0 5.0 X 22 Example 20.9825.40  6.00 0.30 0.90 46.40 Pb:0.02 0.02 43.5 580 15.0 5.0 X 23 Example21.50 28.80  6.00 0.30 0.90 42.50 — 0.00 22.2 580 15.0 5.0 X 24 Example17.57 30.50 12.00 0.10 0.40 39.40 Cr:0.03 0.03 21.5 640 15.0 5.0 X 25Example 20.85 32.50  6.50 0.05 0.50 39.60 — 0.00 144.0 630 15.0 5.0 Y 26Example 18.05 33.50  6.70 0.70 0.55 39.80 Cu:0.70 0.70 33.8 630 15.0 5.0X 27 Example 16.30 35.10  6.70 0.70 1.50 39.70 — 0.00 34.5 630 15.0 5.0X 28 Example 11.59 37.80  6.70 0.80 3.00 40.10 Ti:0.01 0.01 45.5 65015.0 5.0 X 29 Example 11.82 39.10  6.50 0.80 0.73 41.06 — 0.00 72.0 65015.0 5.0 X 30 Example 10.75 40.00  6.00 1.00 0.70 41.50 Sn:0.05 0.0540.1 680 15.0 5.0 X 31 Example  8.00 40.00  5.00 1.00 1.00 45.00 — 0.0040.6 680 15.0 5.0 X 32 Comp. ex.  6.68 45.10  5.00 0.56 0.56 42.10 —0.00 39.6 690 15.0 5.0 X 33 Comp. ex. Commercially available hot dipgalvannealed steel sheet X 34 Comp. ex. Commercially available hot dipAl coated steel sheet X Bold underlines indicate outside scope ofpresent invention or outside preferable range.

[Table 2]

TABLE 2 Fe-Al layer layer + Main layer diffusion Other layer) Fe-Alcontaining com- Mg-Zn (inter- phase (%) pounds containing Results ofevaluation facial Mg-Zn containing phase (%) Fe-Al-Zn FeAl inter- oxideCorro- Hydro- Thick- Total MgZn Mg2Zn3 MgZn2 Total phase phase metalliclayer sion gen ness Area Area Area Area Area Area Area Area Thicknessresist- pene- No. Class (μm) ratio ratio ratio ratio ratio ratio ratioratio (μm) LME ance tration 1 Comp. ex. 22.1   0.0  0.0 0.0 0.0   0.00.0  0.0 0.0  2.1 D C D 2 Comp. ex. 23.5   0.0  0.0 0.0 0.0   0.0 0.0 0.0 0.0  2.1 D C D 3 Example  5.1 79.7  0.0 6.2 73.5  20.3 0.1 20.2 0.0<0.2 B B A 4 Comp. ex.  5.8 85.5 81.2 4.3 0.0 14.5 2.0 12.5 0.0 <0.2 A CC 5 Comp. ex. 25.1   0.0  0.0 0.0 0.0   0.0 0.0  0.0 0.0  3.0 D C D 6Example  5.0 74.6  0.0 0.0 74.6  25.2 0.0 25.2 0.2 <0.2 A A A 7 Example 5.0 74.0 20.5 53.5  0.0 26.0 1.0 25.0 0.0 <0.2 A A AA 8 Example  5.372.9 20.2 52.7  0.0 27.0 1.0 26.0 0.1 <0.2 A A AA 9 Example  5.3 72.447.8 11.6  13.0  27.6 1.5 26.1 0.0 <0.2 A A AAA 10 Example  6.4 68.047.4 12.5  8.1 31.8 2.0 29.8 0.2 <0.2 A A AAA 11 Example  6.8 67.9 45.022.9  0.0 32.1 2.1 30.0 0.0 <0.2 A A AAA 12 Example  6.9 68.1 46.1 22.0 0.0 31.9 0.9 31.0 0.0 <0.2 A A AAA 13 Example  7.2 65.8 44.1 21.7  0.034.1 2.0 32.1 0.1 <0.2 A A AAA 14 Example  7.1 69.1 48.1 21.0  0.0 30.80.0 30.8 0.1 <0.2 A A AAA 15 Comp. ex. 21.0   0.0  0.0 0.0 0.0   0.0 0.0 0.0 0.0  2.8 D C D 16 Comp. ex. 23.1   0.0  0.0 0.0 0.0   0.0 0.0  0.00.0  3.0 D C D 17 Comp. ex. 22.1   0.0  0.0 0.0 0.0   0.0 0.0  0.0 0.0 4.1 D C D 18 Comp. ex. 20.1   0.0  0.0 0.0 0.0   0.0 0.0  0.0 0.0  2.1D C D 19 Comp. ex.  6.1 22.8  0.0 0.0 22.8  65.4 0.0 65.4 11.8   1.1 D CD 20 Comp. ex.  6.0 18.8  0.0 0.0 18.8  68.8 0.0 68.8 12.4   1.4 D C D21 Example  7.8 59.7 51.1 8.6 0.0 40.3 7.8 32.5 0.0 <0.2 A A AAA 22Example  7.7 60.1 50.8 9.3 0.0 39.9 7.7 32.2 0.0 <0.2 A A AAA 23 Example 8.1 54.1 41.0 13.1  0.0 45.9 7.4 38.5 0.0 <0.2 A A AAA 24 Example  9.949.5 49.5 0.0 0.0 50.3 7.9 42.4 0.2 <0.2 A A AA 25 Example 10.8 34.734.7 0.0 0.0 65.3 7.5 57.8 0.0 <0.2 A A AA 26 Example 11.1 33.8 33.8 0.00.0 66.1 7.5 58.6 0.1 <0.2 A A AA 27 Example 12.5 29.1 29.1 0.0 0.0 70.98.5 62.4 0.0 <0.2 A A AA 28 Example 13.8 23.3 23.3 0.0 0.0 76.6 8.1 68.50.1 <0.2 A A AA 29 Example 14.8 20.4 20.4 0.0 0.0 79.6 9.1 70.5 0.0 <0.2A A AA 30 Example 17.5 15.5 15.5 0.0 0.0 84.4 9.0 75.4 0.1 <0.2 A A AA31 Example 20.0 10.0 10.0 0.0 0.0 90.0 10.0  80.0 0.0 <0.2 A B AA 32Comp. ex. 22.0 18.8  0.0 0.0 18.8  70.3 0.0 70.3 10.9   1.3 D C D 33Comp. ex. Commercially available hot dip galvannealed steel sheet D D A34 Comp. ex. Commercially available hot dip Al coated steel sheet A D DBold underlines indicate outside scope of present invention.

The chemical compositions and structures of the plating layers in thehot stamped bodies obtained in the examples and comparative examples andthe various characteristics when hot stamping the plated steel materialswere investigated by the following methods: The results are shown inTables 1 and 2. In Tables 1 and 2, Comparative Examples 33 and 34 relateto the hot dip galvannealed (Zn-11% Fe) steel sheet and hot dip aluminumcoated (Al-10% Si) steel sheet conventionally used as plated steelmaterials for hot stamping and show the results in the case of hotstamping these steel sheets. The chemical compositions and structures ofthe plating layers of Comparative Examples 33 and 34 clearly differ fromthe chemical compositions and surface structures of the plating layersaccording to the present invention, therefore analysis of the chemicalcompositions and structures of these plating layers were omitted.Further, Comparative Examples 33 and 34 are just commercially availableproducts which were evaluated. Therefore, details of the productionmethods of these steel sheets are not known. Further, while not shown inTable 2, the Fe—Al containing phase (Fe—Al—Zn phase and FeAl phase) hasisland shapes. In the islands of the Fe—Al containing phase, the aspectratio was 5.0 or less.

[Chemical Composition of Plating Layer]

The chemical composition of the plating layer was determined bydissolving the plating layer in an acid solution to which an inhibitorinhibiting corrosion of the steel base material was added and measuringthe obtained solution by ICP emission spectrometry.

[Thicknesses of Fe—Al Layer and Oxide Layer]

The thicknesses of the Fe—Al layer and the oxide layer were determinedby cutting out a test piece from the hot stamped body, burying it in aresin, etc., then polishing the cross-section, measuring the imageobserved by an SEM, and averaging the measured values of these in threedifferent fields as the thicknesses of the Fe—Al layer and the oxidelayer.

[Area Ratio and Composition of Each Phase in Main Layer]

The area ratio of each phase in the main layer was determined asfollows: First, a prepared sample was cut into 25 mm×15 mm size. Theplating layer surface was photographed by a 1500×power by an SEM. Fromthe BSE image obtained and SEM-EDS mapping, the area ratio of each phasein the main layer was measured by computer image processing. Theaverages of these measured values at any five fields were determined asthe area ratios of the MgZn phase, Mg₂ Zn₃ phase, MgZn₂ phase, FeAlphase, Fe—Al—Zn phase, and other intermetallic compounds (Mg₂ Si and Al₄Ca). Further, the area ratio of the Mg—Zn containing phase wasdetermined as the area ratio of the total of the MgZn phase, Mg₂ Zn₃phase, and MgZn₂ phase. Similarly, the area ratio of the Fe—Alcontaining phase was determined as the area ratio of the total of theFeAl phase and Fe—Al—Zn phase.

[LME Resistance]

The LME resistance was evaluated by subjecting a sample of the platedsteel material before hot stamping to a hot V-bending test.Specifically, a sample 170 mm×30 mm of the plated steel material beforehot stamping was heated in a heating furnace and taken out from thefurnace when the temperature of the sample reached 900° C. A precisionpress was used to conduct a V-bending test. The V-bending die had ashape of a V-bending angle of 90° and R=1, 2, 3, 4, 5, and 10 mm. TheLME resistance was ranked as follows: Rankings of AAA, AA, A, and B weredeemed passing.

AAA: No LME cracking occurred even with R of 1 mm.

AA: LME cracking occurred with R of 1 mm, but LME cracking did not occurwith R of 2 mm

A: LME cracking occurred with R of 2 mm, but LME cracking did not occurwith R of 3 mm

B: LME cracking occurred with R of 3 mm, but LME cracking did not occurwith R of 4 mm

C: LME cracking occurred with R of 4 mm, but LME cracking did not occurwith R of 5 mm

D: LME cracking occurred with R of 5 mm, but LME cracking did not occurwith R of 10 mm

[Corrosion Resistance]

The corrosion resistance of the hot stamped body was evaluated asfollows. First, a sample 50 mm×100 mm of the hot stamped body wastreated by zinc phosphate (SD5350 System: standard made by Nippon PaintIndustrial Coatings Co., Ltd.), then was coated by electrodeposition(PN110 Powernix Gray: standard made by Nippon Paint Industrial CoatingsCo., Ltd.) by a thickness of 20 μm and was baked at 150° C. for 20minutes. Next, cross cuts (40×√2 mm, 2) were made reaching the baseiron. The coated body was used for a combined cyclic corrosion test inaccordance with JASO (M609-91). The maximum blister widths at eightlocations around the cross cuts after the elapse of 150 cycles weremeasured. The average values of the obtained measured values were foundand ranked as follows: Samples evaluated as A and B were deemed passing.

A: Width of coating blister from cross-cut of 1 mm or less

B: Width of coating blister from cross-cut of 1 to 2 mm

C: Width of coating blister from cross-cut of 2 to 4 mm

D: Red rusting

[Hydrogen Penetration Resistance]

The hydrogen penetration resistance of the hot stamped body was found asfollows: First, a sample of the hot stamped body was stored in liquidnitrogen. Thermal desorption spectroscopy was used to find theconcentration of hydrogen penetrating the hot stamped body.Specifically, the sample was heated in a heating furnace equipped with agas chromatograph and the amount of hydrogen released from the sample upto 250° C. was measured. The measured amount of hydrogen was divided bythe mass of the sample to find the amount of hydrogen penetration. Thiswas ranked as follows: Rankings of AAA, AA, A, and B were deemedpassing.

AAA: Amount of hydrogen penetration of 0.1 ppm or less

AA: Amount of hydrogen penetration of more than 0.1 to 0.2 ppm

A: Amount of hydrogen penetration of more than 0.2 to 0.3 ppm

B: Amount of hydrogen penetration of more than 0.3 to 0.5 ppm

C: Amount of hydrogen penetration of more than 0.5 to 0.7 ppm

D: Amount of hydrogen penetration of 0.7 ppm or more

Referring to Tables 1 and 2, in Comparative Example 1, the Si and Cacontents in the plating layer were small, therefore the acicularAl—Zn—Si—Ca phase was not formed in the surface structure of the platinglayer before hot stamping. It is believed that a barrier layer comprisedof a Ca-based oxide film was not formed at the time of heating in thehot stamping. As a result, at the time of the above heating, Zn and Mgin the plating layer evaporated, a thick Mg—Zn containing oxide layer ofmore than 1.0 μm was formed, an Mg—Zn containing phase was not formed inthe main layer, and the LME resistance, hydrogen penetration resistance,and corrosion resistance were all evaluated as being poor. InComparative Examples 2 and 5, similarly the Al, Si, and/or Ca content inthe plating layer was small, therefore at the time of heating in the hotstamping, no barrier layer was formed and the LME resistance, hydrogenpenetration resistance, and corrosion resistance were all evaluated aspoor. In Comparative Example 4, the Mg content in the plating layer waslarge, the corrosion resistance fell due to the excessive sacrificialcorrosion prevention action or, since the Mg content was large, hydrogenpenetration occurred due to the evaporation of Mg at the time of hotstamping. In Comparative Examples 15 to 18, the cooling of the platinglayer did not satisfy the predetermined two-stage cooling conditions,therefore an acicular Al—Zn—Si—Ca phase was not formed at the surfacestructure of the plating layer before the hot stamping, and, at the timeof the heating in the hot stamping, Zn and Mg in the plating layerevaporated and, as a result, the LME resistance, hydrogen penetrationresistance, and corrosion resistance were all evaluated as poor. Fromthe results of Comparative Examples 15 to 18 and other examples, it islearned that to more reliably form the acicular Al—Zn—Si—Ca phase at2.0% or more of area ratio and form a barrier layer comprised of aCa-based oxide film at the time of the heating in the hot stamping, itis preferable to first cool by a 14° C./s or more or 15° C./s or moreaverage cooling speed from the bath temperature to 450° C., then cool bya 5.5° C./s or less or 5° C./s or less average cooling speed from 450°C. to 350° C. In Comparative Example 19, the Si content in the platinglayer was too high, therefore in the plating layer before the hotstamping, an Mg₂ Si phase (other intermetallic compound in Table 2) waspreferentially formed, the acicular Al—Zn—Si—Ca phase was notsufficiently formed, and, as a result, the LME resistance, hydrogenpenetration resistance, and corrosion resistance were all evaluated aspoor. In Comparative Examples 20 and 32, the Ca content or the Alcontent in the plating layer was too high, therefore in the platinglayer before hot stamping, Al₄ Ca and other intermetallic compounds(other intermetallic compounds in Table 2) were preferentially formed,an acicular Al—Zn—Si—Ca phase was not sufficiently formed, and, as aresult, the LME resistance, hydrogen penetration resistance, andcorrosion resistance were all evaluated as poor. In Comparative Example33 using conventional hot dip galvannealed steel sheet, the hydrogenpenetration resistance was excellent, but the LME resistance andcorrosion resistance were evaluated as poor. In Comparative Example 34using conventional hot dip aluminum coated steel sheet, the LMEresistance was excellent, but the hydrogen penetration resistance andcorrosion resistance were evaluated as poor.

In contrast to this, in all of the examples according to the presentinvention, by suitably controlling the chemical composition of theplating layer and the phases contained in the plating layer and the arearatios of the same, a hot stamped body in which the LME resistance andhydrogen penetration resistance were improved and, furthermore, thecorrosion resistance was excellent could be obtained. From the BSE imageof the SEM of the plating layer surface before hot stamping (and inaccordance with need the SEM-EDS mapping image), in all examples, anacicular Al—Zn—Si—Ca phase was present in an area ratio of 2.0% or moreat the surface structure of the plating layer before hot stamping.

Example B

In this example, the inventors studied the point of change of thecooling speed between fast cooling and slow cooling in two-stage coolingof a plating layer. First, except for using a plating bath for forming aplating layer similar to Example 10, etc. (bath temperature 600° C.),and further changing the point of change of the cooling speed to 375°C., 400° C., 425° C., 450° C., 475° C., and 500° C. and making theaverage cooling speed of the first stage 15° C./s and the averagecooling speed of the second stage 5° C./s, they followed the sameprocedure as in the case of Example A to obtain plated steel materialswith plating layers formed on both surfaces of the steel base materials.They examined the area ratios of the acicular Al—Zn—Si—Ca phases at thesurface structures of the plating layers at the obtained plated steelmaterials. The results are shown in FIG. 4 .

Referring to FIG. 4 , if the point of change of the cooling speed is400° C., the area ratio of the acicular Al—Zn—Si—Ca phase is 1.9%, andtherefore 2.0% or more could not be secured, while if the point ofchange of the cooling speed is 425° C., 450° C., and 475° C., 2.0% ormore of the acicular Al—Zn—Si—Ca phase could be formed. In particular,if the point of change of the cooling speed is 450° C., the highest arearatio of the acicular Al—Zn—Si—Ca phase could be achieved.

REFERENCE SIGNS LIST

-   -   1 plating layer    -   2 oxide layer    -   3 diffusion layer    -   4 steel base material    -   5 interfacial layer    -   6 main layer    -   7 Fe—Al layer    -   8 Mg—Zn containing phase    -   9 Fe—Al containing phase    -   9 a FeAl phase    -   11 α phase    -   12 α/τ eutectic phase    -   13 acicular Al—Zn—Si—Ca phase

The invention claimed is:
 1. A hot stamped body comprising a steel basematerial and a plating layer on a surface of the steel base material,wherein the plating layer has a chemical composition comprising, by mass%, Al: 15.00 to 45.00%, Mg: 4.50 to 12.00%, Si: 0.05 to 3.00%, Ca: 0.05to 3.00%, Fe: 20.00 to 50.00%, Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to1.00%, Sn: 0 to 1.00%, Ti: 0 to 1.00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%,Ni: 0 to 1.00%, Mn: 0 to 1.00%, and balance: Zn and impurities, theplating layer comprises an interfacial layer positioned at an interfacewith the steel base material and containing Fe and Al and a main layerpositioned on the interfacial layer, the main layer comprises, by arearatio, 10.0 to 85.0% of an Mg—Zn containing phase and 15.0 to 90.0% ofan Fe—Al containing phase, the Mg—Zn containing phase comprises at leastone selected from an MgZn phase, Mg₂ Zn₃ phase, and MgZn₂ phase, and theFe—Al containing phase comprises at least one of an FeAl phase andFe—Al—Zn phase and an area ratio of the Fe—Al—Zn phase in the main layeris 10.0% or less.
 2. The hot stamped body according to claim 1, whereinthe chemical composition of the plating layer comprises, by mass %, Al:20.00 to 30.00% and Mg: 5.50 to 10.00%.
 3. The hot stamped bodyaccording to claim 1, wherein the Mg—Zn containing phase comprises anMgZn phase, and an area ratio of the MgZn phase in the main layer is30.0% or more.
 4. The hot stamped body according to claim 1, wherein theMg—Zn containing phase comprises an MgZn phase and Mg₂Zn₃ phase, and anarea ratio of a total of the MgZn phase and Mg₂Zn₃ phase in the mainlayer is 25.0 to 85.0%.
 5. The hot stamped body according to claim 1,wherein the Fe—Al containing phase comprises an FeAl phase and an arearatio of the FeAl phase in the main layer is 5.0 to 55.0%.
 6. The hotstamped body according to claim 2, wherein the Mg—Zn containing phasecomprises an MgZn phase, and an area ratio of the MgZn phase in the mainlayer is 30.0% or more.
 7. The hot stamped body according to claim 2,wherein the Mg—Zn containing phase comprises an MgZn phase and Mg₂ Zn₃phase, and an area ratio of a total of the MgZn phase and Mg₂Zn₃ phasein the main layer is 25.0 to 85.0%.
 8. The hot stamped body according toclaim 3, wherein the Mg—Zn containing phase comprises an MgZn phase andMg₂ Zn₃ phase, and an area ratio of a total of the MgZn phase and Mg₂Zn₃phase in the main layer is 25.0 to 85.0%.
 9. The hot stamped bodyaccording to claim 2, wherein the Fe—Al containing phase comprises anFeAl phase and an area ratio of the FeAl phase in the main layer is 5.0to 55.0%.
 10. The hot stamped body according to claim 3, wherein theFe—Al containing phase comprises an FeAl phase and an area ratio of theFeAl phase in the main layer is 5.0 to 55.0%.
 11. The hot stamped bodyaccording to claim 4, wherein the Fe—Al containing phase comprises anFeAl phase and an area ratio of the FeAl phase in the main layer is 5.0to 55.0%.